It is well known that the texture, flavor, odor and appearance of many food products are significantly affected by the temperatures at which such products are prepared, stored and served. It has also been appreciated that the moisture content, another important factor in the overall desirability or acceptability of food products, is greatly affected by the thermal treatment accorded to such products. Many complex chemical and physical changes in food products are brought about by the manner in which their temperature and moisture content are regulated.
Because of these and other considerations, numerous different apparatus and methods for heating and cooling food products have been disclosed in the prior art. Thus, for example, convection ovens, infrared ovens, microwave ovens and impingement heating ovens have all been utilized, either alone or in combination, to impart thermal energy to food products. Particularly desirable and effective apparatus and methods for imparting thermal energy to food products have previously been disclosed, for example, in U.S. Pat. Nos. 3,884,213 and 4,154,861.
The apparatus and methods disclosed in the foregoing patents employ columnated jets of air or other gaseous fluid to create discrete areas of very high heat transfer on the surface of a food product. This high heat transfer accomplishes the advantageous effects of crisping, browning, searing, chilling or freezing in those localized areas where the jets impinge against the surface of the product. When the jets are of proper size, spacing, temperature, velocity and direction, very desirable results are obtained. Experience has shown that the use of hot air jets impinging substantially perpendicularly to the surface of the product achieves heat transfer rates two to ten times greater than the heat transfer from the same volume of air moving parallel to the product's surface. This very rapid heat transfer is desirable to initiate rapid cooking, baking, cooling or freezing. Furthermore, when the food products are passed beneath the jets, these effects are spread evenly over the food surface.
U.S. Pat. No. 3,884,213 further discloses an apparatus for heating both the surface and interior portions of food products through the use of microwave radiation in combination with impingement heating. When microwave energy is employed in combination with the columnated jets of heated gas impinging on the surface of the food products, internal and external heating can be accomplished simultaneously. However, the apparatus and methods previously disclosed in the prior art have been found to have certain disadvantages which are overcome through use of the apparatus and method disclosed herein.